Fly-ash is usually obtained by separation from gas resulting from the partial or complete burning of carbonaceous material such as coal. It is usually composed of particles having a diameter in the range from 0.1 to 200 .mu.m. If the burning is carried out at elevated pressure it is practice to clean the resulting gas at about the same pressure so that the separated fly-ash is caught in a pressurized vessel.
In order to be able to transport, work up or dump the fly-ash it should generally be depressurized.
Studies on the depressurizing of fly-ash by means of lock hopper systems have shown that the valves between the lock hoppers wear out rather quickly.
It has now been found that this problem can be solved by depressurizing the fly-ash by means of a relatively long and narrow pipe.